Electrical precipitation apparatus



March 5, 1940.

AFiled Aug. 19. 1958 3 Sheets-Sheet 1 March 5, 1940. H. J. WHITElELECTRICAL PRECIPITATION APPARATUS I Filed Aug. 19, 1938 3 Sheets-Sheet2 AfA/em' JI WH/TE, Ma/XVM @M fw/@V 5 Sheets-.Sheet 3 m /fy/ m www H. J.WHITE Filed Aug. 19, 1938 ELECTRICAL PRECIPITATION APPARATUS m d Z W /0.y j e w a i e ow. .1.. .,f W w v m I III I'IIOI 7l .IIS M 5 J W 6 f fL8 fb. u u J u lj 4. I 0 ..9 u. .IH .ml/ l-|| Il.. u 7N u u W 7 V1 3 m Jm E March 5, 1940.

Patented Mar. 5, 1940 2,192,250 l ELEcTarcAL PRECIPITATION APPARATUSHarry J. `White, Los'Angeles, Calif., assignor to Research Corporation,New York, N. Y., a corporation oi' New York Application August 19,1938,' Serial No. 225,809

UNITED STATES PATENT OFFICE Claims.

become electrically charged by attachment of electrons or ions and arethen attracted to an electrically charged member upon which the chargedparticles are collected. It has been common practice to effect chargingof the suspended I, particles by passing them between two opposed becomecharged with the same electrical sign as l .electrodes between which ahigh potential difference is maintained, one of the two electrodes beinga discharge electrode at which there is silent or corona electricaldischarge that ionizes the gas and causes the suspended particles to thedischarge electrode. 'Ihls is termed charg- 5 ing action. The chargedparticles migrate, under the influence of the electric ileld betweenelectrodes, toward the other electrode which is a non-dischargingelectrode of extended surface. and collect or become precipitated uponthe surface of the second electrode, which is consequently termed thecollecting electrode.

In the following description and in the appended claims, the termdischarge electrode" will be understood to designate an electrode thatfacilitates corona discharge therefrom, because it has a congurationthat establishes a suillcient- 1y high potential gradient at or near itssurface to create corona discharge before there is a disruptivedischarge or spark-over. For thispurpose the discharge electrode usuallytakes the form of a member of small surface area, such as a smalldiameter wire or a rod provided with sharp edges or points, whereby theymay be created in the immediate vicinity .thereof a sufficiently highelectric field intensity to cause ionization and corona.` discharge.v'Ihe term non-discharging electrode will be understood to designate anelectrode I that minimizes or prevents corona discharge therefrombecause it has a configuration that establishes a sufliciently low fieldconcen tration at or near the surface to suppress corona discharge atelevated potentials lower than the voltage required' for disruptivedischarge or sparkover. For this purpose. a.V non-discharging electrodeusually is one ofextended'surtace area,

substantially free fromsharp corners or other parts of sharp surfacecurvature at all portions which are located within the electric eld, soas to substantially avoid ionization or corona discharge at thatelectrode.

Dust particles in the gas stream become electrically charged in theionizing field and there migrate to the collecting electrode under theinfiuence of the electrical forces exerted on them, and when theparticles reach and are precipitated on the electrode the electriccharges are neutralized or lost. The layer of dust' particles on theelectrode is exposed ordinarily to the lgas stream which tends to blowoff loose individual particles or agglomerations of particles, theerosion being a result of the gas velocity. .The par-- ticles are heldon the electrode more or less securely against erosion either byelectricalfforces or by the physical properties of the particlesthemselves. When the ,collecting electrode is within the range of thecharging, eld, as in the single field type of precipitator, the eldrecharges and reprecipitates most of the particles that may be blown 01Tthe electrode, but even lthensome particles areblown along the electrodesurface and ofi the outlet end of the electrode when the dust has suchphysical properties thatthe particles offer little resistance to gaserosion. There are some dusts that are particularly hard to collect forthis reason, one typical example being fly ash which is composed of upto some velocity termed the critical velocity above which erosion lossincreases rapidly with an increase in velocity and becomes quiteappreciable. Itis probably true of most dusts, if not all, that there isa critical velocity above which losses are appreciable. The presentinterest centers particularly around dusts lfor which the criticalvelocity is relatively low, since the amount of these dusts carried intothe outgoing gas stream isconsiderable and makes it diflicult to obtainhigh overall collection efliciencies at economic velocities. Obviously,the critical velocity for a given dust limits the capacity of a giventreater if a minimum collection efilciency is to be obtained.

This censeuon amenity is inherent in the dust being collected and hasthe ee'ct of reducing the capacity of a u nit of a given size operatingat a caused by erosion losses increases the size of the plant andequipment required to treat a given volume of gas without falling belowa minimum efiiciency.

As might be expected, it has been frequently observed that the erosionlosses are increased when the collecting' electrode surfaces becomecoated with 'deposited particles. Consequently, the percentage of lossis increased by relatively heavy dust loading of .the gas stream and -bythe usual custom of shaking or rapping the collecting electrode atrelatively long intervals. as these circumstances keep down the totalamount of time when the electrode is clean and therefore the erosionloss is' the least. At present it is common practice to discontinue theflow through the preciptator section being cleaned forthe durati n ofthe rapping period, to permit the. deposi` removed from the collectingelectrode to settle down into the hopper in a quiet zone free of thegas-stream. Obviously, rapping at short intervals with discontinuousflow is not resorted to becahse it reduces the operation time of thatsection and, proportionately, the eiective capacity of the treater. A

It is thus a general object of my invention to' provide an improvedelectrical precipitator in which losses from erosion by the gas streamare minimized or eliminated and the collection efilciency is improved,particularly in the case of dusts having certain physicalcharacteristics making collection vdiiicult.

It is als'o an object of the invention to provide an electricalprecipitator adapted to continuous operation without need fordiscontinuing gas ow during the periods of cleaning, thus improvingefficiency and capacity and eliminating the necessity of providing gasby-pass flues and dampers or an additional precipitator. s

A further object isv to increase the capacity of,

s a precipitator of given size by decreasing erosion lossesk at anygiven gas velocity and so allowing. higher gas velocities, especially inthe case of dusts having low critical velocities in treaters ofconventional types.

Another object is to provide an electrical precipitator that is adaptedto handle a gas stream with a relatively high dust loading without lossof collection enlciency from the accumulation of deposits on thecollecting electrode.

'I'hese and other objects are lattained in a prev non-dischargingelectrode intermediate the discharge and collecting electrodes. diateelectrode forms a part of a system of collecting electrodes, as somedust particles collect the space between the discharge and intermediateelectrodes, in which space an ionizing field is maintained by suitablemeans. Another elec-v nleai nele of -a substantially nen-dischargingExpressed dif- Ierently, the reduction in collection eciencyv 'of Fig.5; and

character is preferably maintained between the intermediate andcollecting electrodes to aid precipitation and to prevent redlspersionof collected material into the gas stream. especially during rappingPeriods.

How the above objects and advantages of my invention, as well as othersnot specifically mentioned, are attained will be more evident from thefollowing description ings, in which:

Fig. 5 is a vertical transverse section through a variational form ofelectrical precipitator constructed according to myinvention, usingplate type electrodes;v l

- Fig. 6 is a vertical longitudinal section through the variational typeof precipitator on line 6-6 Fig. '1.is a horizontal section on line'I-'I of Fig. 6.

Referring now particularly toFlgs. l and 2, I

shall rst describe my invention Ias applied to a precipitator havingfour similar sets of electrodes which, with the exception of thedischarge electrodes, are ofthe tubular type. 'I'he electrodes areenclosed within-cylindrical housing I0 which,

for convenience in manufacture and assembly, is

l preferably made in three. sections, the bottom,

intermediate and top sections being indicated at Illa, Illb, and I0crespectively. Bottom section Illa contains mechanism for collecting andremoving continuously the accumulated dust taken from the gas stream,and this portion of the precipitator will be described in greater detaillater. Inside this bottom section is an outer hopper formed by inclinedplate members II and the housing sidewalls, the hopper being V- shapedin cross section, as shown in Fig. 2. Within and-slightly above the`outer hopper, is

' an inner hopper I2 generally oi' diamond-shape sloping top walls I6completely cover over the topof the inner hopper. Walls Il, I5 and I6ex- -tend horizontally until they intersect the vertical wens ofhousing-section Ina, as shownln Fig.' 1, sothat the housing walls formthe end members of the hopper. As may be seen' best from Fig. 1, gasinlet opening II inthe' housing wall opens Adirectly into the' interiorof hopper I2.

5 Within the upper portion of the housing is, header plate 2li-whichmayconveniently be held between the upper and intermediate sections Iband Inc ofthe housing and which forms in the upper section ofthe housinga gas chamber into f which the' gas from the :electrodes is dischargedbefore leaving the treater through gas outlet 2l, which may be connectedto any suitable gas" con- 1 duit. openings z2 1n the side and top 'fsfthe housing are' normally covered by plates and provide means forobtaining Vaccess to the treater interior for assembly and adjustmentlof parts.

Supported on header plate 20 and located-,centrally of housing I0, isvertically extending inandthe annexed draw-. y

. 10 Fig. 1 is a vertical median section through an sulator 24 whichcarries at its upper end four equally spaced, radially extending arms25. A similar insulator arrangement is placed at the lower end'of thehousing and comprises insulator 21 which is supported centrally `of thehousing on bracket 21a attached to the inside faces of hopper walls I4'.At the upper end of insulator 2l, there are four radially extending arms28 which are vertically aligned with arms 25. Between ea'ch pair ofvertically spaced arms 25 and 28, there is stretched a wire 29 ofrelatively small diameter which forms a discharge electrode. As may beseen in Fig. 3, 4there are thus four such discharge electrodes spacedequally about the center of housing Il).`

Surrounding each discharge electrode 29, is an intermediate electrode 30in the shape vof a cylinder or tube open at both ends. Intermediateelectrodes 30 .are made of screen, perforated metal, or other similarforaminous material that permits the passage of Asuspended particlesfrom the gasstream through the electrodes to the collecting electrode,as will be more fully explained. Each electrode 30 ts snugly around animperfor.

late metal nipple 3| at the upperA end of the electrode and against ring32 on the nipple. Each l nipple 3| is flanged at its upper end, theflange 3|a resting upon the upper surface of header plate 2l) while thenipple itself extends through a suitable opening in the header plate.Nipples 3| are preferably spot-welded to header plate 20 tohold themsecurely in place. The intermediate electrodes are held at their lowerends by similar construction, each tubular screen itting snugly over animperforate metal nipple-34 and resting upon a collar 35 attached tothenipple. Nipples preferably have flanges at their lower ends by whichthey are welded or otherwise attached to top walls i6 of inner hopperi2, the nipples opening into the interior of the hopper. .The fourcollecting electrodes 38 are cylin- -drical `members of sheet metal, ofsomewhat greater diameter than the intermediate electrodes 30, supportedat their upper ends by horizontal plate 39 which is suspended by meansof insulators 40 from header plate 20. Collecting electrodes 38 areconveniently supported from plate 39 by forming flanges 38a. on theupper ends of the electrodes which are easily spot-welded to the plate..This holds the electrodes against undesired movement. Collectingelectrodes 38 are held against lateral movement at their lower ends bytransverse brace 42 which extends across the interior of the housing,and to which all the collecting electrodes are fastened as by welding-The upturned ends of brace 42 are provided with adjusting screw andclamping nut assemblies 43 which extend three horizontally elongatedslots therein and bear against insulators 44 on the housing wall, thuselectrically insulating .the electrodes from the housing and at the sametime permitting close lateral adjustment of the electrodes. I

'Ihe electrodes thus far described are' arranged in four independentgroups operating in par-4 allel, each group comprising a tubularcollecting electrode 38 and a tubular intermediate electrode 38 arrangedconcentrically of each other and coaxially of a discharge electrode 28.Pour groups are placed in a single housing to obtain larger capacitythan can be had with a single group which is the basis u'nitof thetreatcr; and any number of electrode groups may be uled according to thetotal las capacity dlired. As will become apparent, each group ofelectrodes may be considered as comprising a discharge electrode and acollecting electrode system that includes the outer impervious tube uponwhich the bulk of the dust is collected and the inner pervious tube uponwhich a portion of the dust is collected, but which is referred to as anintermediate electrode because of its position between the dischargeelectrode and the outer collecting electrode.

The rapping mechanism for cleaning the electrodes of. accumulateddeposits of precipitated particles yincludes two separate hammer systemsthat may be independently operated, there being one for the collectingelectrodes and one for the intermediate electrodes. The first of thesehammer systems adapted to rap the collecting electrodes consists of atransverse'shaft 48 journaled at opposite ends in suitable bearings 58mounted g on the housing walls. At the mid-point of the shaft, isfastened hammer 5| which extends radialb from the shaft a suillcientdistance to strike mers; and by turning handle 55 in a directiontorotate shaft 48 counter-clockwise through a semi-circular arc, as viewedin Fig. 2, hammer 5| is brought into contact with the other anvil 52 andthe second pair of electrodes may be rapped. In order to effectivelyinsulate the collecting electrodes from the housing, shaft 481s made ofelectrically-non-conducting material such as wood that insulates metalhammer 5| from metal bearings `5|) and housing I0, althoughit'will beunderstood that it is not necessary that shaft 48 be one continuousinsulating member, for it may also be a composite member having thenecessary insulating sections. The same results may be secured by makingthe arms of hammer 5| of insulating material, or placing insulatingsections elsewhere in the system.

A similar rapping mechanism is provided for jarring the intermediateelectrodes to remove any accumulations on them. 'I'his second rappingmechanism comprises shaft 58 journaled in suitable bearings on the wallsof housing section |8a and having a. handle 59 on one end externally ofthe housing for oscillation of the shaft. Hammer 58 is fastened to theshaft midway of its length at the proper' position to strike against thetwo U-shaped anvils Gl'which are supported one from each top plate I8 ofinner hopper l2 adjacent a pair of nipples 34. I shaft 58 throughasemi-circular arc by means of handle 55' causes hammer 60.150 strikeagainst anvils 8| alternately. The impact against an anvil vibrates thetop wall i8 and nipples 34 and transmits the vibration to electrodes 38from which accumulated dust is shaken by the jarring action. It lwill beunderstood that. although both rapping mechanisms are here shown asbeing manually operated for. the sake of simplicity in the descriptionand drawings, it is fully within the contemplation of the invention tomechanically operate both rapping mechanisms by any suitable type ofmechanical drive. Particular- 60 Oscillation of ly is this so. becausemechanical operation of the i ation may'be desired.

rapping mechanisms oers the advantage of automatic actuation of thehammers at short intervals, such as 30 or v60 seconds when the type of ydust being precipitated out of the gas stream is such that betterprecipitation is obtained by cleaning the electrodes at frequentintervals.. Obviously, the rapping mechanisms may be actuated at longerintervals when the character of the dust collected is such as to makethis operation satisfactory. and inV this case manual oper- Dust shakenfrom the inside surfaces of electrodes 80 `and fallingdown inside theelectrodes, passes through nipples 34 into hopper I2 and collects in anarrow trough 6l at the bottom of the hopper. 'I'he dust from the insidesurfaces of collecting electrodes 38 and that dust from electrodes 30which falls von the outside of them, drops out of the electrodes on tothe inclined hopper walls I 6 and collects at the bottom of hopper II ina narrow trough 65. The dust in lthese two troughs is removed by asingle continuous chain 68 which passes over idler sprocket 69 anddriven sprocket 10, the latter being driven Aby motor 1I or any othersuitable source of power. As viewed in Fig. 1, the upper run of chain 68moves towards the right and carries with it the accumulated dust intrough 64, the dust being carried intothe housing around idler sprocket69V until it builds up suiliciently that it is carried out of thehousing by the lower run of chain 68. This dust, together with the dustaccumulated in trough 65 is then carried tothe left,'as viewed in Fig.l, by the bottom run of chain 68 and discharged from theV precipitatorhousing through discharge opening 12, from which the dust fallsinto anysuitable collecting means, such as a bin or other conveyor. There ispreferably suillcient slack in chain 68 that it scrapes or drags alongthe bottom surface of both troughs 64 and 65, the troughs being butlittle wider than the chain itself, -so that the links of the chain keepthe troughs scraped clean. The

movement of chain 68 is preferably continuous, and its speed should bedetermined by the rate at which dust accumulates in hoppers I I and I2.

The electrical conductor 15 connecting the discharge electrodes 29 to asource of high tension continuous or unidirectional current, indicatedgenerally at A, is brought into the precipitator housing throughinsulator 16. A second conductor 11 is brought' into the precipitatorthrough insulator 18 and is used to connect the collecting electrodes 38with a suitable sourceof high tension unidirectional or continuouscurrent, indicated 'generally at B.- Ihe precise nature of theelec- Ytrical means used to supply these high tension currents is not essentialto the present invention, and it is not shown because such means arewell known in the art and may take different forms depending uponvarious engineering consideraof corona discharge before a disruptiveorspark 'alegando discharge takes place between the electrodes '28 and Il.-The charging or ionizing field main? tained between electrodes 2l and80 is of a character to charge electrically any particles carriedthrough the field bythe ga's stream, the'charge taken on by theparticles being of the same sign or polarity as the discharge electrode.

The electrical apparatus also maintains an added non-discharging fieldbetween'the interemediate and collecting electrodes, by maintaining apotential difference between electrodes 86 and 28. Since theintermediate electrodes are grounded, the collecting electrodes 38 aremaintained at a polarity opposite to that of the disthe discharge andintermediate electrodes. As

typical Vofvoltages which may be applied to the o electrodes, butwithout any limitation thereto, v

because the actual' voltages used in any given instance will be dictatedby sound engineering design, it has been found in practice advantageouswith a collecting electrode diameter of 8" and an intermediate electrodediameter .of 6, to apply a potential of from 35,000 to 50,000 volts tothe discharge electrode and apotential of opposite polarity of from12,000 to 18,000 volts to the collecting electrode. In accord withstandard practice, itis preferred that the discharge electrodebenegative and that the collecting electrode be' positive.- Thus, theconductor 15 is shown connected conductor 11 connected to the' positiveterminal of source B, both'of said sources having their other terminalsgrounded as indicated at 8 0.

During operation of the precipitator, a stream of gas carrying suspendedparticles to be separated or precipitated from the stream enters theltreaterthrough inlet I1 from a suitable gas conduit, not shown, whichAcanbe attached to the housing by means of the flange around the openingI1. Gas entering inlet I1 is introduced directly into inner hopper I2which acts as an inlet header to distribute the gas to the four nipples34 through which the gas ilows into the ionizingl ields. As the gasflows upwardly 'in a direction generally parallel to the electrodesurfaces, the suspended particles are -removed by' the electricalprecipitating action previously described, the clean gas then leavingthe electrodes through nipples 3l and entering the space at the top ofthe 'treatein e The clean gas then leaves the treater through outletopening 2| and may be wasted to the atmosphere, or the gas may beconducted away for further use by means of a suitable gas conduitattached to the flange around opening 2 I It will be noticed that, asthe gas leaves hopper I2, each nipple 34 directs an individual gasstream upwardly through one set of vertically extending electrodes.thestream being more orvv less confined throughout its entire verticaltravel by the electrode130. Becauseintermediate electrode 38 ts snuglyaround nipple 34 at the bottom end of the electrode, v-all of the gas isini tially introduced into the ionizing -ileld from y 25. previouslychargedin the ionizing eld b etween to the negative terminal of sourceA, and thev 2,199,250v the nipple 34. Likewise, none of the gas can 1 30to conilne the gas stream to the space inside the electrode, causessubstantially all the gas to flow in one main stream directly throughthe ionizing iield, though a minor portion of it passes outwardly'through the foraminous electrode 30 into .the annular space between theintermediate l and 'collecting electrodes, circulating in and outthrough the intermediate electrode as a result of electric wind eiects.This advantageous con- 'struction' is permitted because the intermediateforaminous electrode is grounded. As pointed out above, the housing isgrounded, and grounding the intermediate electrode eliminates 4thenecessity of electrically-insulating these two members. Consequently, itis possible to connect some gas impervious directing member, such as Qhopper i2 and nipples 3l, or header plate 20,

to the housing walls, and at the same time have that member engagedirectlythe intermediate electrodes at their ends where gas enters orleaves theionizing field. The advantage is that all the incoming gas isintroduced directly into the ionizing fields, andfnone can ilrst enterdi rectly the space behind the tubular electrodes where the gas streamwould not be exposed to the ionizing iield.

Although the annular space between the inter,- Imediate and collectingelectrodes is open at both the top and bottom of the collectingelectrode 38, there is little or no tendency to create an eddy currentof gas leaving the collecting electrodes and ilowing into the housing.Consequently, the annular space surrounding each intermediate electrodeconstitutes a relatively qui'- escent zone in which there is little orno gas movement.

The, ionizing iield radiating outwardly from a discharge electrodeterminates in part' where it encounters the metal wires or portions ofthe opposed intermediate electrode 30; but where the ionizing fieldencounters an opening in the intermediate electrode, the iield projectsor continues on through the opening and terminates on collectingelectrode 33. The separate eld inde- Dendently maintained .between theintermediate and collecting electrodes is effective in the annular spacebetween these two electrodes, and is superimposed on the parts of theionizing ileld extending beyond the intermediate electrode. As a resultof the fact that the total area .of the Openings in electrode 4il!) isnormally considerably-in excess of the area of 'the metal portions of`the electrode, and as a result of the field distribution mentioned, thebulk of the charged particles are carried on throughy the openings inthe intermediate electrode under the action of the electric fields andare deposited upon the inside surface of collecting electrode 38. Aportion of the particles strike the intermediate electrode and becomeneutralized, thus kbecoming precipitated upon the intermediateelectrode. A part of the particles so precipitated continue to adhere tothis electrode, while a part of them eventually are carried on throughthe openings in the electrode and are precipitated upon the collectingelectrode. It is because of the Iact that both the cylindricalelectrodes oppose the -rate of accumulation of dust deposits.

tated upon their surfaces that the two .of them may be considered asconstituting a collecting electrode system which i's made up of twoseparate electrodes that are electrically distinct and. insulated fromone another, but both of which .operate to facilitate precipitation ofthe charged particles.`

The two rapping mechanisms for cleaning the intermediate and collectingelectrodes may be operated either mechanically or manually at suchintervals as may be necessary according to the It is a particularadvantage of this form of treater that 'the rapping may be carried outatvery short intervals, of the order of 30 to 60 secondsif desired, togive the eiect of a continuous shaking or cleaning action ontheelectrodes, because the gas ilow through any one of the electrodes. neednot be discontinued during the cleaning operation. As mentioned, above,the largest part of the 'precipitated particles collect eventually uponthe inside surface i' a collecting electrode 38, and when they arejarred oi the surface they fall downwardly within the annular space be'tween the collecting and intermediate electrodes which is a quiet zoneArelatively free from gas ow. Consequently, the dust deposits from thecollecting electrodes 38 and 'from the outer surfaces of theintermediate electrodes 30 fall down-A wardly out of the electrodesystem without being picked up by the upwardly moving gas stream, andare collected in the outer hopper I l which is entirely separated fromthe incoming gas. The dust deposits shaken loose from the inner sur-Afaces of the intermediate electrodes tend to fall downwardly throughnipples 34. However, this dust is brought into contact with the upwardlyrising gas stream. The larger agglomerates of precipitated particlessettle down within hopper l2, but a few of the smaller agglomerates orindividual particles are again picked u'p and carried suspended in thegas stream.. These resuspended particles are again subjected to thecharging and precipitating action of the electric elds Within theelectrodes and vwill eventually either be precipitated on the collectingelectrode surfaces or will accumulate ln large agglomerates which willfall down in hopper l2 to the trough 64 at the bottom of the hopper.Because of this improved operation of the precipitator constructed asdescribed, the electrodes may be rapped at such-frequent intervals as tobe kept in a substantially clean condition at all times efficiency oi'the precipitator with respect to fly ash, which is a dust that isextremely diilcult to collect because of the spherical shape andnon-adherent character of its particles, it was found that the loss ofdust from the outlet end of a single tubular collecting electrodewithout any intermediate electrode 30, amountedto 10% of the dustVintroduced at the inlet end oiE the electrode at a volume o f 80 cubicieet per minute of gas. Insertion of a foraminous intermediate electrodebetween the collecting and discharge electrodes decreased the dust lossfrom the precipitator to 1% at the same gas volume. That is.

fss

I at the same capacity the quantity of dust carried in the outlet gasstream was reduced to one-tenth its former value. It was also found thatthe capacity of the same precipitating unit could be increased to 190cubic feet per minute without exceeding the former loss of 10%, that is,with .a collection eiliciency of 90%. A'Ihis last experb ment wascarried out with both the 'collecting and intermediate electrodesgrounded so that no precipitating'iield was maintained independentlybetween'them. y

The effectivenesse'of the non-discharging eld is emphaticallydemonstrated by a later set of tests using a slightly higher potentialat the discharge electrode, in which the capacity at 90% eiiiciency wasdetermined tol be 215 cubic feet per minute with both -the intermediateand colcollecting electrodes grounded.` When a high potential wasapplied to the collecting electrode. it was found that the capacity ofthe treater could be increased to 465l cubic feet per minutewithoutVdropping the collection eflicien'cy below 90%, and that the eiliciencywas increased to 98% at the initial ow of 215 cubic feet per minute. Allthese tests were -run on fly ash becausethat material is'hard tocollect. Other tests show that the increase in capacity or efllciencydepends o`n the kind of dust treated, beinggreatest for those materialsthat are hard to collect in a conventional precipitator of the singlenem type.'

Experiments on various other types of dust uniformly show a markedincrease in capacity or improvedgcollection eiliciency lor both as aresult of having a foraminousI intermediate elec- 'trode between thedischarge and collecting electrodes to form radjacent the collectingelectrode a quiescent zone in which the dust is collected orAprecipitated onl the collecting electrode and which is suilicientlyfree from disturbance by the gas stream that there is little or notendency for .o the dust to be redistributed into the gas stream evenwhen the electrodes are .being shaken for cleaning. 'Ihe tests alsoshowuniformly a still greater increase i'n lcapacity at v the samecollecting eiliciency, on a .greater increase inoelciency at the samecapacity, as a result ol.' maintaining an independent substantially.non-discharging 'eld betweenthe intermediate and collecting electrodeswhich facilitates pre-l cipitatton of the particles on the collectingelectrodes-and amplies the effectiveness of the collecting electrodesystem comprising two'separate electrodes.

Flgs. 5, 6 and 7 illustrate a variational form of precipitator which isfundamentally the same as the .precipitator valready described, exceptthat not shown in the.' drawings as. devices are' the intermediate andcollecting. electrodes instead of being' tubular in shape are nowplatelike in shape'and other minor changes in thehoppers in which thedust falls after being shaken from the electrodes.; The dust may beremoved v from hopper bottombf housing 85. by vany suitable type of,conveyor or -valve mechanism.; whichl ls,

welllmow in' the art.

'.'rne typical prooipitacor' illustrated fmthe drawings is composed oi'two'groupsof electrodes,

' fine the entering'gas stream to the .space be- `vertical portions ofmargins 95 of electrodes 94 guasto each group comprising discharge'.intermediate,

' and collecting electrodes and each group being a substantial duplicateof the'other group. The discharge electrodes 98 are a-series "offwlresof relatively small diameter arranged in a'overtical 6 and to maintainthe discharge electrodes of the two groups in parallel spaced verticalplanes. This supporting framework is mounted upon insulators 93 carried`on brackets attached tothe walls of housing 85 in order to electricallyinsulate the discharge electrodes and their support ing structure fromthe housing walls', which are grounded in accordance with commonvpractice. The discharge electrodes may, of course, be held in place byany other' suitable structure electrically insulated from Vthe housingand the other electrodes. I i

On each side of ra seriesof discharge electrodes and parallel to theplane thereof, is a foraminou`s intermediate electrode. which is in theform of a sheet or plate. Intermediate electrodes 94 mayconveniently besuspended from the top wall of housingBS and are connected directlythereto since it is -preferred'th'at these electrodes be grounded.through their eieotriai connection to the housing. -The intermediateelectrodes may be of any foraminousconstruction, such as is afforded bythe use offwire screen or perforated metal sheets; and arev preferablybounded on their vertical and bottom sides by an imperforate border 95which strengthens the electrodes against flexing and helps direct andcontween the 4discharge-andinternjlediate electrodes.A For'thevlatterreason, borders 95 are gas impervious and at thevertical edges of theintermediate electrodes 94 are shaped toprevent anyl direct ilow of theincoming' gas stream into thev space behindvthe electrodes, thus theonly gas behind the electrodes lisv that which vpasses through theperforate portions. o1 electrodes. after having once entered the ionizedeld. The

adjacent the side walls ofthe housing arebent` through an angle'of 90and joined to the verti- 'cal side walls in a suitable gas tight manner.

In the central portion of-the precipitatorl the vertical portions ofmargins 95 of two electrodes 9i are formed in a U-shape, thus connectingtogether two intermediate electrodes into the forni of a hollowenvelope. `This construction is shown best in Fig. 7. Borders 95 extendupwardly and,-

are connected to the roof of the housing in order the electrode, butmaybe left open at the bottom in'A order to facilitate removal of dustfrom the electrodes. since gas, entry at this point is negligible 1inany event. 'As a consequence of this construction,- the'main AstreamVoi' gas flows parallel to' theelectrode surfaces and through the spacebetween the discharge ,and 'intermediatev i electrodes where theionizing eld is maintained.

bomomeaouorfurtneroonnnmnnem'doww n "4' to prevent gas fromleaking'around the edge of alternately against the lower edges of thetwo the horisontal passages between electrodes 94. A

sidiary baffles 98 of limited vertical extent mayY be placedadvantageously between the two intermediate electrodes on opposite sidesof a series of discharge electrodes, as shown inFig. 5. The top edges ofthese subsidiary baffles 88 are placed at the same level as the topedgesof baflles 91,

thatis, lnot within arcing distance of electrode framemembers 89, butthe subsidiary bailles extend downwardly only to the bottom of marginalportions 95 of the intermedite electrodes. Main baffles 91 greatlyimpede any gas flow directly into the lower portion of the housing fromthe regions at the ends of the electrodes, especially from inlet 86,while the subsidiary baffles act as deflectors that substantiallyeliminate anyv tendency for the gas to flow downwardly into the hopperbottom of the housing from the spaces between the electrodes. In thismanner a quiescent zone is assured not only at the sides of theintermediate electrodes removed 'from the discharge electrodes, but alsoin the lower portion ofthe housing into which the dust falls when it isjarred from the electrodes.

The collecting electrodes are suspended from thetop wall of housing 85by insulators |0| and 'are yheld against lateral movement at their lowerends by insulators |02 which are attached to'bafiles .91. In this waythese electrodes |00 are electrically insulated from the frame and fromthe discharge electrodes in order that a separate potential may beapplied to the collecting electrodes. The collecting electrodes are inthe' form of imperforate plates extending parallel to the intermediateelectrodes and the planes of the discharge electrodes, there being acollecting electrode at each side of the precipitator lbetweenintermediate electrode 94 and the side wall of the housing, while `atthe center a single collecting electrode |00 may be used for bothintermediate electrodes, since both faces of the collecting electrodeare available for collecting surfaces. A

Thus, as in the form o'i the invention previously described, aforaminous non-dischargingelectrode is provided intermediate eachdischarge electrode and an opposing collection electrode' trodes whichit raps, and the hammers are cf'suf" flcient length that oscillation ofshaft |05 through a semi-circular arc causes the hammers to strikeintermedite electrodes, as shown in Fig. 5. Dust jarred loose from theseelectrodes falls down into the hopper bottom of housing 85.

Similar rapping mechanism is provided for the collecting electrodes andcomprises a shaft ||0 llocated midway between each pair of collectingelectrodes |00 and journaled in bearings mounted on thehouslng walls andbailles 91. A suitable number'of. hammers ||2 are secured to shaft ||0and the hammers upon oscillation of shaft ||0 through a semi-circulararc, strike against the lower edges of the collecting electrodes. Oneshaft ||0 and its attached hammers,

is used to clean two collecting electrodes, as the hammers strikealternately against the electrodes; and where an odd number ofcollecting electrodes is used as shown in Fig. 5, the hammers arestaggered alongv the length of the two shafts ||0 in order that thehammers will not interfere with each other if they swing simultaneouslytowards the same electrode. Shaft l0 is' either made ofnon-conductingmaterial. or

rapping mechanism during the cleaning opera-A tion. Although nooperating mechanism is shown, it will be understood that the severalrapping mechanisms may be actuated by either mechanical or manual meansattached to shafts |05 and ||0 and adapted to oscillate these shafts atsuitable intervals of time.

Suitable electrical apparatus and connections, not shown, are employedto apply a high potential current to the electrodes and maintain betweenthe discharge and intermediate electrodes an ionizing field as a resultof the corona discharge around electrodes 88. A separate and independentelectric fleld is preferably maintained between the intermediate andcollecting electrodes, but this field is of a substantiallynondischarging character. It is preferred that the intermediateelectrode be grounded and that the discharge and collecting electrodesbe maintained at opposite potentials or polarities in order to maintainboth the fields in the direction that forces charged particles towardthe intermediate and collecting electrodes.

In general, the operation of the last described form of my invention isthe same as described in connection with Figs. l to 4; and thecollecting electrode .systems comprising a foraminous intermediateelectrode and a plate collecting electiode oier the same advantages ofincreased capacity and collecting efficiency as compared with a singleplate collecting electrode, that `are found in the use of electrodes oftubular shape. Since in this form of precipitator the gas stream isdirected horizontally through the treater, the lower portion of thehousing is free from disturbance caused by the incoming vgas streamflowing through it as through hopper |2, and the collection of dustfalling off the electrodes is somewhat simplified, only a single hopperbeing necessary in this case.

In the first form of the invention described f that the collectingelectrodes be imperforate; but in the second form described, it may bedesired -above utilizing tubular electrodes, it is preferred v undersome circumstances to replace the plate type collecting electrodes withperforate or gas pervious electrodes, as for example, with the rodcurtain" type of collecting electrode which is 45 intermediateelectrodes ina ymore easily by rapping than is the yplate type A A clesto be removed, the combination of a discharge electrode; a substantiallynon-discharging collecting electrode; va foraminous substantiallynon-discharging electrode intermediate the discharge and collecting.electrodes; means for 2. maintaining a nigh potential ionizing meldbetween the discharge and intermediate electrodes; means forindependently maintaining a non-dis-l charging precipitating ileldbetween the intermediate and collecting electrodes; and means for w'directing the main stream of gas into,'throu'gh,

and out of the space between the discharge and intermediate electrodesin a direction sub stantially parallel to the electrode surfaces.

ilows a stream oi' gas carrying suspendedV particles to be-removed, thecombinationof a discharge electrode: a substantially non-discharg"- ingcollecting electrode; a foraminous substantially non-discharginggrounded electrode intermediate the discharge and collecting electrodes:means for maintaining a, high potential ionizing field between thedischarge and intermediate electrodes; means for independentlymaintaining a non-discharging precipitating eld between v theintermediate and collecting electrodes with the collecting electrode ofva polarity opposite to that of the discharge electrode; and means fordirecting the main stream of gas into.- through. and out of the spacebetween the discharge and direction substantially parallel .to theelectrode surfaces. l

3. In an electrical precipitator lthrough which iiows a stream of gascarrying suspended parti-` -cles to be removed, the combination oi' adischarge electrode: a. substantially non-,dischargfing collectingelectrode; a foraminous substantially non-discharging electrodeintermediate the discharge andl collecting electrodes; means for 5'5independently maintaining ahigh potential ionizing iield between thedischarge vand intermedi-- ate* electrodes: means for maintaining aseparate non-discharging precipitating field the intermediate andcollecting electrodes, said precipitating 'iield being maintained in aldirectlonto .assist movement of charged particles toward the.collecting electrode to collect thereon:

and means for directing the main stream of gas f inta-through, and outof the space between the 65 discharge and intermediate electrodes ina.direc tionv substantially parallel to the electrode sur-j i faces.-

4. man electric nreeipiiair through which nows a stream of gascarryingsuspended particles to be removed, the combination of adischargejelectrode; a-'collecting electrode system comprising apair ofsubstantially non-discharging electrodes of which one is a foraminouselectrode positioned-between the discharge electrode 2. In an electricalprecipitator through which and the other non-discharging mi forindependently maintaining a high potential I `ionizing field between thedischarge electrode and the collecting electrode system; means formaintaining a separate non-discharging precipitating ileld between thepair of non-discharging 5' -quiescent zone between the electrodes of thecollecting electrode system. f

5. In an electric precipitator through which flows a stream of gascarrying suspended particles to be removed,` the combination of a dis-15 charge electrode; a collecting electrodesystem comprising a pair ofsubstantially non-discharg- I I ing electrodes of which one is aforaminous grounded electrode `positioned between the dischargeelectrode and the other non-discharging electrode; means for maintaininga high potential ionizing field between the discharge electrode j andthe collecting electrode system; means for maintaining a separatenon-discharging precipitating held between the pair of non-dischargingelectrodes with the polarity of said other nondischarging electrodeopposite to that of the discharge electrode; and means directingsubstantially all the gas iiowv into, through', and out of the spacebetween the discharge' andforaminous velectrodes in a directionsubstantially parallel to the electrode surfaces, and taining arelatively quiescent zone between the electrodes ofA the collectingelectrode system: 35

6. In an electric precipitator through which flows a stream of gascarrying suspended particles to be removed, the combination of adischarge electrode; a collecting electrode system comprising a pair ofsubstantially non-discharging ,electrodes of which one is a foraminous49 grounded electrode positionedl between the discharge electrode andthe other non-discharging electrode; means for maintaining a highpotential ionizing neld between the discharge electrode" and thecollecting electrode system; means for 45 maintaining a separatenon-discharging precipitating neld between the pair of non-dischargingelectrodes: and means directing substantially all the gas ilow into,through, and out of the space -between the discharge and foraminouselectrodes 50 in a ldirection substantially parallel to the elec` trodesurfaces and maintaining a relatively quiescent zone between electrodesof the collecting '1. In an electric precipitator through which 55 flowsasti-'cam of gas carryingl suspended parti. cles to beremoved, thecombination of a discharge electrode: a collecting electrode systemsurrounding the discharge electrode and comprising a pair ofsubstantially non-discharging ,v

cylindrical. c'o-aidal electrodes 'of which one-is an imperforateelectrode, and the other is a foraminous grounded electrode between thedischarge electrode and the imperforate electrode; -meens fermniniaininga high potential vionizing 6 neld between the dischargeelectrode and the collecting electrode system; means for maintainingaseparate non-discharging precipitating eld, bev tween the foraminousvand imperforate electrodes in a direction to assist movement of chargedpar- 'I ticles toward the imperforate electrode;` and means directingsubstantially all the gasy flow into, through. and `out of the spacebetween the dischargefand'foraminous electrodes in a direc- .electrodesurf' 75r f aces and maintaining a relatively quiescent zone l betweenthe electrodes of the collecting electrode system.

8. In an electric precipitator through which ows a stream of gascarrying suspended particles to be removed, the combination of adischarge electrode; a collecting electrode system comprising a pair ofsubstantially non-discharging electrodes of which one is a. ioraminouselectrode positioned between the discharge electrode and the secondnon-discharging electrode; means for maintaining a high potentialionizing iield between the discharge electrode and the collectingelectrode system; means for maintaining a separate non-dischargingprecipitating iield between the pair of vnon-discharging electrodes;means directing substantially all the gas`ow throughthe space betweenthe discharge and foraminous electrodes and maintaining a relativelyquiescent zone between the electrodes oi the collecting ,electrodesystem; rapping mechanism for jarring deposited dust particles from saidsecond non-discharging electrode; a hopper to catch dust. jarred fromsaid second non-discharging electrode; a second rapping mechanism forjarring deposited 'dust particles from the foraminous electrode; asecond hopper to catch dust jarred from the foraminous electrode; andconveying means for removing dust from both hoppers.

9. In an electric precipitator through which flows a stream of ga'scarrying suspended particles to be removed, the combination of adischarge electrode; a collecting electrode system comprising a pair oi'substantially non-discharging cylindrical, co-axial electrodes of whichonev is an imperforate electrode, and the other is a foraminous groundedelectrode positioned bef tween the discharge electrode and theimperiorate electrode; means for maintaining a high potential ionizingfield between the discharge electrode and the collecting electrodesystem; means-for maintaining a separate'non-discharging precipitatingiield between the foraminous and imperforate electrodes in a directionto assist movement of charged particles toward the imperforateelectrode; means directing substantially all the gas flow through thespace between the discharge and ioraminous electrodes and maintaining arelatively quiescent zone between the electrodes of the collectingelectrode system: rapping mechanism for jarring deposited dust particlesfrom the imperforate electrode; a hopper to catch dust jarred from theimperforate electrode; a second rapping mechanism for jar ring depositeddust particles from the foraminous electrode; a second hopper to catchdust jarred from the foraminous electrode; and conveying means forremoving dust fromrboth hoppers.

10. In an electric precipitator through which I ows a 4stream of gascarrying suspended particles to be removed, the'combinationof adischarge electrode; a collecting electrode system comprising a pair ofsubstantially non-discharging electrodes of which one is an imperforateelectrode, and the other is a foraminous electrode between the dischargeelectrode and the' imperforate electrode; means formaintaining a highpotential ionizing iield between the dischargel electrode and thecollecting electrode system; means for maintaining a separate non-dis,-

charging precipitating eld 'between the foraminous and imperforateelectrodes; means directing substantially. all the gas flow through thespace between the discharge and foraminous electrodesand maintaining arelatively quiescent zone between the electrodes of the collectingelec'- trode system; rapping mechanism for :larring deposited dustparticles from the imperforate elecl trode; 'an outer housing enclosingall said electrodes and having a hopper bottom adapted to catch dustjarred from the imperforate electrode: a second rapping mechanism forjarring deposited dust particles from the foraminous electrode; an innerhousing forming a combined gas inlet header and hopper adapted to catchdust jarred from the foraminous electrode; .and conveying means forremoving dust from both hoppers.

1l. In an electric precipitator through which flows a stream of gascarrying suspended particles to be removed, the combination of adischarge electrode; a collecting electrode system comprising a pair ofsubstantially non-discharging electrodes of which one is a. foraminouselectrode positioned between the discharge electrode and the othernon-discharging electrode; rapping mechanism for jarring deposited dust`particles. from the collecting electrode system; an outer housingenclosing al1 said electrodes and having a hopper bottom adapted tocatch'dust jarred from said other non-discharging electrode; an innerhousing forming a combined gas inlet 'header and hopper adapted to catchdust jarred from the foraminous electrode; and conveying means forremoving dust i'rom both hoppers.

12. In an electric precipitator through which ows a stream of gascarrying suspended particles to be removed, the combination of a diseAcharge electrode; a collecting electrode system comprising a pair ofsubstantially non-discharging electrodes of which one is a foraminouselectrode positioned between the discharge electrode and the othernon-discharging electrode; rapping mechanism for jarring deposited dustparticles from the collecting electrode system; an outerhousingenclosing all said electrodes and having a hopper bottom'adaptedYKto catch dust jarred from said other non-discharging electrode; aninner housing forming a combined gaslnlet header and hopper adapted tocatch dust jarred from the foraminous electrode; and a continuous `chainconveyor having" an upper run moving dust from the inner hopper into theouter hopper, and a lower run removing dust from the outer hopper.

13. In an electrical precipitator through which flows a stream of gascarrying suspended particles to be removed, the combination of agrounded housing; a discharge electrode; a substantially non-dischargingcollecting electrode; a foraminous substantially non-dischargingelectrode intermediate the discharge and collecting electrodes; meansfor maintaining a high potential ionizing field between the dischargeand intermediate electrodes; means for independently maintaining aseparate non-discharging precipitating field between the intermediateand co1- lecting electrodes; and gas impervious members connected to thehousing and engaging the ends of the intermediate electrode where thegas enters and leaves the ionizing'eld to direct the main stream of gasinto, through, and out of 'the' space between the discharge andintermediate electrodes in a direction substantially par-7 allel to theelectrode surfaces.

14. In an electric precipitator through which" ilows a stream'of gascarrying suspended particles to be removed, the combination of a dis.

drical, substantially non-discharging electrodes, lof which one is animperforate electrode and the other is a foraminous electrode positionedbetween th discharge electrode-and the imperforate electrode; an outerhousing enclosing all said electrodes and having a hopper bottom adaptedto catch dust falling from said imperforate electrode; an inner housingconnected to the outer housing and forming a. combined gas inlet headerand hopper adapted to catch dust falling inside said foraminouselectrode; and a nipple connected to the hopper and engaging chargeelectrode; a collecting electrode system comprising a pair ofsubstantially non-discharging electrodes-of which one is a foraminouselectrode positioned between the discharge electrode and the othernonfdischarging electrode; an out# er housing enclosing all saidelectrodes .and having a hopper bottom adapted to catch dust falll ingfrom said other non-discharging electrode; an inner housing connected toVthe outer hous-U ing andforming a combined gas inlet header and hopperadapted to catch dust falling inside said foraminous electrode; and anipple connected to the'hopper and engaging the lower end of theforaminous electrode to direct the stream of gas upwardly into the,space inside the foraminous electrode.,

. I HARRY J. WHITE.

